The present invention relates to nanometerology and in particular to systems and methods for deconvolving tip effects associated with scan data in scanning probe microscopy.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, x-rays, etc.) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. Exposure of the coating through a photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Due to the extremely fine pattern dimensions employed in present day integrated circuits, techniques are being generated to help accurately measure such dimensions. One generic class of tools employed for such high accuracy measurements is the scanning probe microscope (SPM). Generally, scanning probe microscopy provide pictures of atoms on or in surfaces, thereby providing atomic level surface imaging. One form of a Scanning Probe Microscope is an Atomic Force Microscope (AFM), which is sometimes alternatively referred to as a Scanning Force Microscope (SFM). AFMs include a sensor with a spring-like cantilever rigidly mounted at one end and having a scanning tip at a free end. AFMs may operate in contacting and non-contacting modes. In the contacting mode, the tip of an AFM is placed in low force contact with a surface of a semiconductor wafer or other workpiece of interest. The workpiece is then displaced relative to the AFM in one or more directions in a plane (e.g., the tip contacts the workpiece in a Z axis while the workpiece is displaced in the X and/or Y directions), to effect a scanning of the workpiece surface. As surface contours or other topographic features are encountered by the tip during scanning, the cantilever deflects. The cantilever deflection is then measured, whereby the topography of the workpiece may be determined.
In non-contacting operation, the tip is held a short distance, typically 5 to 500 Angstroms, from the workpiece surface, and is deflected during scanning by various forces between the workpiece and the tip. Such forces may include magnetic, electrostatic, and van der Waals forces. In both the contacting and non-contacting modes, measurements of a workpiece topography or other characteristic features are obtained through measuring the deflection of the cantilever. Deflection of the cantilever may be measured using precisely aligned optical components coupled to deflection measurement circuitry, although other techniques are sometimes employed.
Another form of SPM is a Scanning Tunneling Microscope (STM). Where a feature of interest is non-topographic, AFMs as well as STMs may be utilized used to measure the workpiece feature. Examples of non-topographic features include the detection of variations in conductivity of a semiconductor workpiece material. An AFM can be used to scan a workpiece in the non-contacting mode during which deflections in the cantilever are caused by variations in the workpiece conductivity or other property of interest. The deflections can be measured to provide a measurement of the feature. STMs include a conductive scanning tip which is held in close proximity (within approximately 5 Angstroms) to the workpiece. At this distance, the probability density function of electrons on the tip spatially overlap the probability density function of atoms on the workpiece. Consequently, a tunneling current flows between the workpiece surface and the tip if a suitable bias voltage is applied between the tip and the workpiece. The workpiece and tip are relatively displaced horizontally (in the X and/or Y directions) while the tip is held a constant vertical distance from the workpiece surface. The variations in the current can be measured to determine the changes in the workpiece surface.
In another mode of operation, an STM can be used to measure topography. The scanner moves the tip up and down while scanning in the X and/or Y directions and sensing the tunneling current. The STM attempts to maintain the distance between the tip and the surface constant (through moving the tip vertically in response to measured current fluctuations). The movements of the tip up and down can be correlated to the surface topography profile of a workpiece.
In both types of SPMs, the dimensions of the scanning tip is important. By knowing the dimensions of the tip, correction factors may be used to correct scan data to thereby remove the impact of the scanning tip itself from the measurement, in order to improve the accuracy of the measurement. Such tip dimensions are typically determined by taking physical measurements of the tips or using some type of known calibration standard. Such processes may be burdensome and are static, meaning they do not account for scanning tip changes over time, for example, due to wear. There is a need in the art for more systems and methods for ascertaining scanning tip effects.
The present invention relates to a system and method of determining a tip dimension associated with a scanning probe microscope.
In accordance with one aspect of the present invention, a method of deconvolving the effects of a scanning tip in a scanning probe microscope is effectuated using multiple, different type scanning tips to scan an artifact such as a feature. For example, by scanning a feature with a conical type scanning tip, a feature width associated with a top portion of the feature is ascertained without knowledge of the conical tip dimensions. The feature is then scanned by another scanning tip such as a cylindrical type tip to generate a scan profile associated therewith. Using the top feature portion information gathered by the conical tip, and taking the cylindrical tip scan profile into account, the width of the cylindrical tip may be determined readily. In addition, once the cylindrical scanning tip is determined, the radius of curvature associated with the conical tip may be determined.
In accordance with another aspect of the present invention, a system for deconvolving the effects of a scanning tip is disclosed. The system comprises a scanning probe microscope having one or more cantilevers and a scanning tip(s) associated therewith. The system further comprises a processor which is adapted to monitor a deflection of the cantilever as the scanning tip is scanned across an artifact such as a feature and generate a scan signal associated with the scanning position and deflection of the scanning tip/cantilever assembly. The processor is further adapted to take the scan signals associated with multiple scans of the artifact using different types of scanning tips to deconvolve the tips effects of the various type tips in the respective scan signals.